Although substantial progress has been made on understanding the mechanism of HIV-1 entry into cells, much less is known about the process of virus transmission in vivo. What structural and functional properties of the viral Env protein differentiate the transmitted virus from other variants and facilitate transmission across mucosal surfaces? In this application, we propose to characterize genetically and biologically HIV-1 virus populations isolated from both the donor and recipient immediately following a transmission event in a unique cohort of discordant couples in Zambia. These studies will provide a unique opportunity to investigate the virologic determinants of heterosexual transmission specified by the variable regions of gp120 in biologically relevant viral envelope genes using samples from a large, well-characterized discordant couple cohort that represents the predominant subtype (C) of HIV-1 worldwide. Our hypothesis is that the extreme genetic bottleneck that we have observed, which appears to select for viruses with more compact, neutralization sensitive envelope glycoproteins selects for a virus, which has biological properties that confer unique advantages for transmission and establishment of infection. Partners in concordantly positive couples, particularly those where both are infected by different viruses, are at high risk for superinfection and subsequent virus recombination. We hypothesize that risk of superinfection will depend on virus diversity, will be enhanced by acute infection in one partner, will reflect an inability to immunologically defend against the incoming virus, and that studies of these events will inform on the breadth of protection conferred by immunity to natural infection. For these studies we will follow prospectively, in both Rwanda and Zambia, both partners of couples where we have documented infection of the seronegative partner by a genetically unrelated virus from that in their spouse and monitor for superinfection. Specifically we will: 1. Determine which biological properties of subtype C newly transmitted variants could facilitate establishment of infection in a new host and correlate these with structural features of Env that characterize these isolates, 2. Determine whether a genetic bottleneck occurs in the genital compartment of subtype C infected donor partners or if there is a biological restriction of the transmitted virus population in the genital compartment of the recipients, and 3. Determine the frequency, kinetics and the virologic and immunologic ramifications of HIV superinfection in both partners following acute/early infection. The results of the proposed studies, which are aimed at characterizing the biological properties of newly infecting HIV-1, the origin of the genetic bottleneck observed in acutely infected individuals, and the details and consequences of HIV-1 superinfection, will enhance our understanding of the heterosexual transmission process and will yield novel information that is critical to the design and testing of globally effective vaccine candidates.